Reserve for losses and loss expenses (Components of reserves for losses and loss expenses) (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Jun. 30, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Dec. 31, 2014
Insurance Loss Reserves [Abstract]
Case reserves	$ 1,295,385		$ 1,278,697
IBNR	1,740,602		1,717,870
Reserve for losses and loss expenses	$ 3,035,987	$ 3,122,717	$ 2,996,567	$ 3,172,285	$ 3,192,663	$ 3,243,147